Optical pressure sensor

Abstract

An optical pressure sensor, such as a microphone, is constituted by two membranes, but where the sound does not arrive perpendicular to the membrane, but comes in from the side. The membranes may be parallel as in a Fabry-Perot or slightly skew as in an Air-wedge shearing interferometer. The pressure sensor uses interferometric readout, and consists of two membranes with essentially equal characteristics, where at least one of the membranes is partially transmitting and partially reflective and the other membrane is at least partially reflective, the membranes being separated by a cavity defined by a spacer part, where the distance between the membranes is variable to provide a shift sensitive Fabry-Perot resonator, and where the two membranes have a common back volume being sealed or essentially sealed in the frequency one wish to measure, and where a pressure increase results in that the distance between the membranes move in opposite directions.

The invention claimed is: 
     
       1. An optical pressure sensor using interferometric readout of pressure variations, the optical pressure sensor comprising:
 two membranes with essentially similar mechanical characteristics, wherein at least one of the membranes are partially transmitting and partially reflective, and where the second membrane is at least partially reflective, the membranes being separated by a cavity defined by a spacer part maintaining a distance between the membranes, the membranes providing a interferometer; 
 a readout unit for sensing the variations in the distance between the membranes; and 
 wherein the cavity defines a first volume between the membranes is essentially sealed from the second volume outside the membranes, one of them representing a reference volume and the other being a sensor volume being subject to pressure variations so that a pressure variation in one of the volumes results in a change in the distance between the membranes, the membranes moving in opposite directions. 
 
     
     
       2. The optical pressure sensor according to  claim 1 , wherein the cavity defined by the membranes have an opening on at least one side for receiving the pressure variations thus constituting the sensor volume, the propagation direction of the pressure variations thus being parallel with the membranes. 
     
     
       3. The optical pressure sensor according to  claim 1 , wherein the cavity between the membranes communicates with the pressure variations in the environment, thus constituting the sensor volume while the volume outside the membranes is enclosed in the reference volume. 
     
     
       4. The optical pressure sensor according to  claim 1 , wherein the cavity between the membranes constitutes at least a part of the reference volume while the volume outside the membranes constitute the sensor volume being subject to pressure variations. 
     
     
       5. The optical pressure sensor according to  claim 1 , the sensor and membranes being made from two symmetrical parts, each defining one membrane and a recess, the membranes and recesses defining a cavity between them. 
     
     
       6. The optical pressure sensor according to  claim 5 , wherein the symmetrical parts are machined from silicon discs, the membranes being produced from e.g. silicon nitride. 
     
     
       7. The optical pressure sensor according to  claim 1 , wherein one of the membranes are tilted relative to the other, so as to provide an interference pattern in the light having interacted with both the membranes, and where at least part of the variations in the pattern resulting for the pressure variations is read by the readout unit. 
     
     
       8. The optical pressure sensor according to  claim 7 , wherein the membranes are produced on silicon wafers, and where the tilt is obtained by machining/etching a height difference in one end, depositing the membrane material and then etching the membrane free from the back side of the wafer, whereby the strain in the membrane material stretches the membrane so that the membrane is provided with a skew angle relative to the original orientation of the surface. 
     
     
       9. The optical pressure sensor according to  claim 7 , wherein the membranes are produced on silicon wafers, and wherein the tilt is made by pulling part of one of the membranes using electrostatic forces. 
     
     
       10. The optical pressure sensor according to  claim 1 , wherein the membranes are parallel, so as to provide a Fabry-Perot interferometer, at least part of the variations in the pattern resulting from the pressure variations is read by the readout unit. 
     
     
       11. The optical pressure sensor according to  claim 1 , wherein the spacer part is adapted to adjust the distance between the membranes and thus the work point of the interferometer. 
     
     
       12. The optical pressure sensor according to  claim 1 , wherein the readout unit includes at least one light source transmitting light toward the membranes and at least one detector receiving light transmitted or reflected from the membranes, so as to measure changes in the received light depending on the variations in distances between the membranes. 
     
     
       13. The optical pressure sensor according to  claim 1 , wherein the readout unit includes an optical element, e.g. a lens, being used for providing light slightly diverging or converging along the optical axis essentially perpendicular to at least one of the membranes so as to provide an interference pattern after having interacted with the two membranes, at least part of the interference pattern being read by at least one detector. 
     
     
       14. The optical pressure sensor according to  claim 1 , wherein the membranes are produced on silicon wafers, including recesses being machined or etched into the wafer the recesses of the two membranes partially overlapping in the central part of the membranes, so as to obtain at least three different distances between the membranes being read optically. 
     
     
       15. The optical pressure sensor according to  claim 1 , wherein the membranes are made on silicon wafers, one or more recesses being machined or etched into the wafers before depositing the membrane material, and wherein the recesses give shape to the produced form of free etched membranes, so that several different distances are obtained between the two membranes, being read optically. 
     
     
       16. The optical pressure sensor according to  claim 15 , wherein the membranes include one or more grooves with sharp angles in top and bottom are used being positioned around the recesses adapted to provide height membrane distances, and this way stiffen the area around the recesses, making the recessed area relatively flat. 
     
     
       17. The optical pressure sensor according to  claim 16 , wherein the grooves are filled with a suitable material to increase the stiffness. 
     
     
       18. The optical pressure sensor according to  claim 1 , wherein the membranes are made on silicon wafers in which grooves are etched without sharp angles or edges within the area chosen for producing a membrane, so that the membrane when deposited and etched free will stretch, reducing the strain in the membrane. 
     
     
       19. The optical sensor according to  claim 18 , wherein an adsorption unit is integrated in the volume used for gas detection, and wherein air is pumped or sucked through the adsorption unit for a given time, where after the flow is stopped and the adsorbed gas is released, and whereby an analysis is performed on the released gas. 
     
     
       20. A gas sensor including a pressure sensor according to  claim 1 , wherein a gas is present in the sensor volume, the gas sensor including a pulsed or wavelength modulated radiation source with a chosen wavelength being able to excite a specific gas to be detected, readout unit being adapted to detect pressure variations at the frequency of the pulsed radiation indicating the presence of the specific gas. 
     
     
       21. The gas sensor according to  claim 20 , wherein a semi permeable membrane is used as an acoustic low pass filter letting the gas through, suppressing outside noise in to the sensor, as well as reducing the leak of photoacoustic signal to the environment. 
     
     
       22. The gas sensor according to  claim 21 , wherein the detection volume between the semipermeable membrane and pressure sensor is made, and where this detection volume is provided with several windows, so as to utilize several different electromagnetic radiation sources on the same gas sample. 
     
     
       23. The gas sensor according to  claim 21 , wherein the detection volume is made in the same silicon disc as the pressure sensor, but where the detection volume is coupled to the sensor volume through a channel, and wherein the walls in the detection volume transmits the chosen electromagnetic radiation used for analyzing the gas. 
     
     
       24. The gas sensor according to  claim 21 , including a microphone and a loudspeaker outside the semipermeable membrane, and in which active noise reduction is used for generating anti-sound, so that the sound amplitude at the semipermeable membrane is reduced, primarily at the frequency or frequency range used for the photoacoustic gas detection. 
     
     
       25. The gas sensor according to  claim 21 , in which an acoustic notch filter is positioned outside the semipermeable membrane, and where the opening into the reservoir volume in the passive notch filter is covered by a gas tight membrane being thin and flexible enough not to affect the acoustic signal, and making sure that the gas does not diffuse into the resonator volume. 
     
     
       26. The gas sensor according to  claim 25 , wherein the membrane is heated so that the sensor may be used at low temperatures without altering the response of the acoustic filter. 
     
     
       27. The gas sensor according to  claim 21 , wherein the semi permeable membrane admits gas into the reference volume so as to provide pressure equalization for static pressure and for frequencies lower that the detection frequency. 
     
     
       28. The gas sensor according to  claim 27 , wherein the semi permeable membrane lets through as much pressure to the back volume as to the volume between the membranes, so that external noise affects both sides the same amount and reduces the contribution from external noise, especially on the detection frequency. 
     
     
       29. The optical pressure sensor according to  claim 1 , where two such gas sensors are used in parallel, but where one analyses the gas without gas flow while the other adsorbs the gas from the gas flow, and after a given time changing mode, the first adsorbing while the other analyzing. 
     
     
       30. The optical pressure sensor according to  claim 1 , wherein an increase in pressure results in an essentially equal change in position for both membranes. 
     
     
       31. The optical pressure sensor according to  claim 1 , wherein a unit is used for active adjustment of the pressure in the sensor back volume so as to adjust the distance between the membranes and provide an optimal work point.